1. Field of the Invention
This invention relates to a color copying machine or electrophotographic copier.
2. Prior Art
A conventional color copying machine shown in FIG. 1 comprises a charging system, a scan-exposure system, a developing system, a transfer system, a paper feed system and a fusing-exit system. More specifically, the scan-exposure system comprises a platen 1, a lamp 2, reflection mirrors 30 to 35, lens 4, and a color separator 5 having three filters of blue, green and red. The developing system comprises a photoconductive drum 6, three developing devices 7, 8 and 9 containing toners of yellow, magenta and cyan, respectively. The transfer system comprises a transfer drum 11 having a gripper 10 for holding a record sheet of paper on the transfer drum 11, and a transfer corotoron 12 mounted in the transfer drum 11. The paper feed system comprises trays 13 and 14 for holding record sheets of paper of different sizes, respectively, and paper feed rollers 15. The fusing-exit system comprises an endless belt 16, fuser rollers 17, exit rollers 18 and an exit tray 19.
FIG. 2 shows a timing chart for the sequential operation of the copying machine. For copying a color original placed on the platen 1, the photoconductive drum 6 is charged uniformly, and the original is illuminated by scanning through the high-intensity lamp 2 and projected to the photoconductive drum 6 through the blue filter of the color separator 5 to form an electrostatic image (FIG. 2a) thereon. Then, the yellow toner (Y) is attracted by this electrostatic image to develop it (FIG. 2b). Then, the developed toner image is transferred to the paper on the transfer drum 11 (FIG. 2c). According to the same procedure, the second (green filter) and third (red filter) scannings are sequentially effected, so that a second image of magenta toner (M) and a third image of cyan toner (C) are transferred onto the paper on the transfer drum 11. Finally, the toner image is fixed to the paper by the fuser rollers 17.
As shown in FIG. 2, the scanning starts at time t.sub.0, and at time t.sub.1 the leading edge of the original image on the photoconductive drum 6 reaches a transfer point T where the photoconductive drum 6 and the transfer drum 11 are held in contact with each other (FIG. 2-(b)). The transfer drum 11 is rotated at the same peripheral speed as the photoconductive drum 6 when the above scanning starts, and the record sheet of a selected size is fed from the paper feed system and is held around the transfer drum 11 by the gripper 10. At time t.sub.1, the leading edge of the record sheet also reaches the transfer position T (FIG. 2-(c)), and then as the photoconductive drum 6 and transfer drum 11 are continued to be rotated at the same peripheral speed, so that the yellow toner image is transferred to the record sheet. Then, according to the same procedure, the magenta and cyan toner images are sequentially transferred to the record sheet. And the toner images are fixed by the fuser 17, so that the full color image is produced on the sheet.
In FIG. 2, character L designates amounts corresponding to the peripheral length of each of the photoconductive and transfer drums 6 and 11. The returning of the scan system (the leftward movement of the lamp 2 in FIG. 1) to its initial position after the scanning operation is hereinafter referred to as "scanning-back". Since the yellow, magenta and cyan toner images are sequentially transferred onto the single record sheet to achieve a color copying, each toner image must be transferred accurately to the same position on the record sheet to achieve a copying of good quality. This requires that drive mechanisms for the scan system, the photoconductive drum 6 and the transfer drum 11 must be operated in synchronism with one another. In view of this, the photoconductive drum 6 and the transfer drum 11 are connected together through a gearing with little backlash so that they are rotated together with each other. And, the drive mechanism for the scan system comprises a servomotor which has a high responsiveness in speed control. With such drive system, the peripheral length L of the transfer drum 11 is so determined that one set of scanning and scanning-back with respect to a record sheet of the maximum size, i. e., A3-size, is effected per one revolution of the transfer drum. Alternatively, the peripheral length L is equal to the length of A-3 size record sheet. Since the transfer drum 11 is rotated at a constant peripheral speed which is equal to that of the photoconductive drum 6, in the former case, the copyings on B5-size to A3-size sheets require the same time whereas in the latter case, the copying on A3-size sheet requires time almost twice as long as that for the copying on B5-size or A4-size sheet since the transfer drum 11 has to be idled one revolution when the lamp 2 is returned to its initial position for the next scanning.